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Smooth Muscle Cells
Smooth muscle tissue is found in many different body systems,
including as part of organs in the digestive, respiratory, and
reproductive tracts and in the walls of blood vessels. Smooth muscle
cells are approximately the same size as cardiac muscle cells and also
have only one nucleus. However, smooth muscle cells are not branched
and, unlike both cardiac and skeletal muscle, smooth muscle cells don't
have sarcomeres. Smooth muscle cells form layers that are usually
arranged so that one runs parallel to an organ and the other wraps
around it. These two muscle layers then contract in turn, causing
alternating dilation and contraction or lengthening and shortening of
the organ, moving substances through internal passages. This is called
peristalsis and is displayed in the process of digestion as food moves
through the gastrointestinal tract.
Similar to skeletal and cardiac muscle cells, smooth muscle can undergo
hypertrophy to increase in size. Unlike the other two muscle types,
mature smooth muscle cells can also divide to produce more cells, a
process called hyperplasia. This can be observed in the uterus, which
responds to increased estrogen levels by producing more uterine muscle
cells.
Smooth muscle cells also do not possess T tubules and do not have a
very extensive sarcoplasmic reticulum. Smooth muscle has actin and
myosin but they are not organized into sarcomeres, so there are no
obvious bands or striations. Instead, actin and myosin is organized
into dense bodies attached to the sarcolemma, shortening the muscle
cell as thin filaments slide past thick filaments. Thin and thick filaments
are aligned in a diagonal pattern across the cell so that contraction
produces a twisting or corkscrew motion, rotating one way as it
contracts and the other way as it relaxes. Cross-bridge formation and
filament sliding processes are the same in smooth muscle as they are in
skeletal and cardiac muscle. Actin, myosin, and tropomyosin are all
present, but smooth muscle cells do not possess troponin as their
regulatory protein. Instead, a molecule called calmodulin binds to
calcium and activates myosin cross-bridge formation. There is also a
greater ratio of actin to myosin in smooth muscle, meaning that there
are more thin filaments for every thick filament.
Most smooth muscles must function for long periods without rest, so
their power output is relatively low, but contractions can continue
without utilizing large amounts of energy. This occurs because the
ATPase in myosin works at a relatively slow rate, meaning that high
levels of ATP are not available for powerful contractions but a steady
supply is produced for sustained contractions. Smooth muscle can also
maintain contractions through a latch state, during which actin and
myosin remain locked together, or latched, in the absence of Ca2+ ions.
This does not require ATP, thereby producing sustained contractions
without using energy. This allows smooth muscles to keep your blood
vessels partially contracted for your entire life without them fatiguing.
Similar to cardiac muscle, smooth muscle is not under voluntary
control. In addition to spontaneous stimulation, smooth muscle can be
stimulated by pacesetter cells that are similar to pacemaker cells and
trigger waves of action potentials in smooth muscle. The autonomic
nervous system or hormones can also stimulate smooth muscle.
Neuromuscular junctions are not present in smooth muscle,
but varicosities, enlargements along autonomic nerves, release
neurotransmitters into synaptic clefts. Smooth muscle can respond to a
variety of neurotransmitters to produce different effects at different
locations.
Smooth muscle can be divided into two types based on how
depolarization and muscle contraction occur. Single-unit smooth
muscle cells contain gap junctions, which allow the cells to be
electrically coupled. Electric couplings allow action potentials to spread
quickly from one cell to the next, permitting coordinated depolarization
and contraction. In this manner, groups of muscle cells act as a single
unit, contracting in unison. This type of smooth muscle is found in
hollow organs, including the gastrointestinal tract, and in the walls of
small blood vessels, and it is often stimulated spontaneously or by
stretching, to produce an action potential.
Multiunit smooth muscle cells rarely possess gap junctions, so they
are not electrically coupled. Stimuli for multiunit smooth muscles come
from autonomic nerves or hormones but not from stretching. This type
of tissue is found in the walls of large blood vessels, in the respiratory
airways, and connected to hair follicles (to make your hair "stand up”),
among other places.